1. Field of the Invention
This invention relates to a method and apparatus for coating a substrate with a layer of polymerized material by simultaneous glow discharge polymerization and sputtering.
2. Description of the Prior Art
Glow discharge polymerization is a well-known technique for preparing a layer of organic or inorganic polymer. There are two basic types of processes. In the first type, the surface of an existing material is polymerized and "cured" by exposing it to a glow discharge which is created in air or inert gas. Surface molecules are activated by the glow discharge and form links and cross links with adjacent molecules. Because the activation is confined to a region near the surface, the bulk of the material is unaltered. In the second type of process, a layer of polymerized material is deposited on a substrate by operating a glow discharge in a monomeric gas adjacent to the substrate. Reactive species created in the glow discharge deposit on the substrate and form a polymerized layer. The polymerization extends throughout the deposited material. This invention is related to the second type of polymerization process.
In a typical polymerization situation of the second type, the glow discharge is established by applying an electric potential across two electrodes provided in a chamber containing a monomeric gas at a pressure less than atmospheric pressure. There is no glow and only a minute current flows through the gas unless the inter-electrode potential exceeds a threshold value which is sufficient to ionize or "break down" the gas.
As is well known, this breakdown potential depends upon the composition of the gas, the system pressure and the distance between the electrodes. After breakdown has occurred, the gas is conductive and a stable plasma can be maintained over a wide range of currents. An established plasma can be maintained with a potential lower than the breakdown potential. The exact composition of the discharge plasma is not known. It is believed to consist of electrons, ions, free radicals and other reactive species.
Several other methods of establishing a glow discharge in a gas are known. One of these which has been used to deposit polymerized material involves producing a radio frequency current in a coil which encircles a chamber containing the gas. If the current is sufficiently high, the gas will ionize and a plasma can be maintained.
However the glow discharge is established, the reactive species created by it will deposit on any nearby surface. Unreacted monomers will also deposit. This simultaneous deposition allows the formation of a polymerized layer which has uniform properties. Surface bombardment by ions, electrons or electromagnetic radiation originating in the plasma may impart additional molecular cross linkages to the deposited layer.
The reactive species are formed in collisions between monomeric molecules and electrons or ions. Thus, the rate of polymerization can be increased by increasing the discharge current. However, as the current is increased, the potential must be increased and the probability of arcing is greatly increased. Arcing is to be avoided because it may destroy or otherwise degrade the polymeric layer and possibly the power supply.
The material deposition rate is directly related to the pressure of the monomeric gas. An increase in this pressure increases the rate of deposition of the monomer and the rates of formation and deposition of the reactive species. Conventional glow discharge polymerization can be accomplished at commercially reasonable rates only by operating at pressures greater than about 100 millitorr (mT). Such a high pressure is a distinct disadvantage when it is desired to perform several vacuum processes simultaneously or sequentially in the same chamber, because many vacuum processes require pressure of less than 50 mT.
It is also known to increase the deposition rate by magnetically concentrating the plasma and directing it toward the substrate. Such a method for depositing polystyrene is described in U.S. Pat. No. 3,297,465. If the substrate is a conductor, it can be made an accelerating electrode. If the substrate is an insulator, the substrate can be placed behind a perforated electrode through which the reactive species can pass. Such a method for forming a layer of polystyrene is described in U.S. Pat. No. 3,318,790. The disadvantage in directing the plasma toward the substrate is that bombardment by ions and electrons tends to increase the temperature of the coating and the substrate. Overheating of the polymeric film is undesirable because its physical and chemical properties tend to become non-uniform. Further, excessive cross linkages may form which will result in a brittle film. Excessive heating may also disassociate or otherwise degrade the substrate.
Sputtering is also a well-known technique for forming a layer of material on a substrate. In sputtering, material is removed from the surface of a target by ionic bombardment and deposited on the substrate. When the material to be sputtered is an electrical conductor, a dc potential is used. When the material to be sputtered is an insulator, it is preferred to use an rf potential in order to eliminate the build-up of surface charges on the insulator and the resulting loss in accelerating potential.
In the conventional process for depositing a layer of a polymer by sputtering, the polymer itself is first prepared in the form of a sheet or powder. A target is fabricated by placing the sheet or powder in contact with a conductive surface. The target and a second electrode are placed in a chamber which can be filled with an appropriate pressure of an ionizable inert gas such as argon. A polymeric coating on a substrate can be formed by establishing a glow discharge by applying a suitable potential between the target and the second electrode. Such a process for depositing Teflon polytetrafluorethylene (PTFE) is described in U.S. Pat. No. 3,767,559.
It is also known to deposit a layer of a chemical compound by reactive sputtering in which one constituent is sputtered in the presence of a gas of another. For example, a layer of metal oxide is deposited by sputtering the metal in a chamber containing oxygen.
Sputtering is generally considered to be a slow process. The deposition rate can be increased by increasing the current and the gas pressure but with the same limitations mentioned above for glow discharge polymerization. Further, if the pressure is increased too much, the deposition rate begins to decrease because sputtered atoms are scattered in collisions with the gas before they reach the substrate.
The sputtering rate can be greatly increased by confining the plasma to a region adjacent to the target. Such confinement intensifies the plasma and increases both the probability of gas atom-electron collisions and the probability that the ions thus formed will strike the cathode. By use of a magnetic field, the plasma can be confined to a region which is only slightly separated from the cathode. One such device is the planar magnetron sputtering cathode which is described in U.S. patent application Ser. No. 532,807, now abandoned, filed Dec. 16, 1974 by R. L. Shrader and K. N. Tsujimoto.